Osseous Engagement of Sacropelvic Porous Fusion–Fixation Screws

(1) Background and introduction: High-demand lumbosacral fusions are often supplemented with sacral-alar-iliac (SAI) screws. The idealized SAI trajectory was estimated to traverse 35 mm of sacrum before crossing the sacroiliac (SI) joint. However, there is debate on how much osseous purchase SAI screws achieve. The goal of this study was to determine the amount of osseous engagement achieved using a porous fusion–fixation screw (PFFS) when placed in a stacked SAI configuration. (2) Materials and methods: We retrospectively reviewed 40 consecutive patients who underwent sacropelvic fixation with stacked PFFS at our institution from 1 June 2022 to 30 June 2023, using intraoperative computed tomography (CT)-based computer navigation. A snapshot of each screw was taken and the length of purchase within the sacrum and ilium was measured on the axial image along the anterior and posterior aspect of each screw. Nineteen patients did not have adequate images available for review and were excluded. (3) Results: The overall mean anterior sacral engagement was 38.6 mm (±8.2 mm), which was found to be statistically significantly greater than the hypothesized threshold of 35 mm (p < 0.001), while posterior sacral engagement was 28.1 mm (±8.6 mm), which was not found to be statistically significantly greater than the hypothesized threshold of 35 mm (p = 1). The mean difference in sacral engagement between the anatomical location for the cephalad screws was 10.3 mm (p < 0.001) and 10.6 mm (p < 0.001) for the caudal screws. The total sacral surface area available for bone ingrowth for bilateral stacked PFFS was calculated to be 3338.3 mm2, while the total iliac surface area available for bone ingrowth was 4364.8 mm2. A mean difference in surface area availability between anatomical locations was −689.5 mm2 (p < 0.001) for the sacrum and 689.5 mm2 (p < 0.001) for the ilium. (4) Discussion and conclusions: The SAI trajectory screws in this cohort of patients achieved approximately 39 mm of sacrum engagement anteriorly and 28 mm posteriorly. This is consistent with prior estimates based on the idealized SAI pathway through the sacrum. PFFSs allow for simultaneous sacropelvic fixation and SI joint fusion, which may reduce the incidence of de novo SI joint pain in patients with long fusion constructs.